Strange distortion problem

[Adam M.]

I am running an LM3886 per national's spec sheet with +-37v rails, 10,000uf cap per rail.

I have a slightly odd problem... anytime I get an output signal larger than 2v, I start hearing static through the speaker. Its not terribly smooth either, about 1.8 or 1.9v I can hear distortion, and then at 2v comes static.

I have the power coming into the chip via alligator clips from the transformer for now, and the ground leads of the capacitor also connect to the 0v point on the transformer, which I use as power star ground.

I have a short wire soldered to the ground pin on the chip, the mute pin is soldered to the bare end of the wire, and alligator clips from the -input and power ground both connect here as tightly as possible. I have tried an input through a pot and direct, my source is a wavetek signal generator.

DC offset is .045v

I don't have a scope to look at the output unfortanately...

Any ideas on what would cause this?

THanks,
Adam

[carlmart]

Originally posted by Adam M.

I have a short wire soldered to the ground pin on the chip, the mute pin is soldered to the bare end of the wire, and alligator clips from the -input and power ground both connect here as tightly as possible.

There should be a resistor, and preferably a bypass cap, between mute pin and -V.


Carlos

[Adam M.]

I have a 47k resistor between mute and ground, forgot to mention it. no bypass cap though. Would that cause the problem I am seeing, if so what value would you suggest?

[janneman]

The mute resistor must go to V-, it is there to bias the input stage. See the chip's internal diagram in the data sheet. Not having the input stage bias would explain the problem.

Jan Didden

[Adam M.]

I mis-spoke in the earlier post, I have mute going to v-, through the 47k resistor...to be specific I have the mute resistor soldered to the -Vin pin of the chip. The resistor I have to ground comes from the inverting input. (I made the mute resistor to ground mistake earlier and that caused 2 days of frustration trying to figure out it wouldn't work) THanks for the response though, this resource has been very valuable.

[janneman]

Adam,

The best you can do at this point is to sketch a diagram from your construction (don't look at the data sheet!) scan it and post it. We've all been down that road. Chances are you find the problem even before you finish the sketch.

Jan Didden

[Adam M.]

Attached is a .gif of the circuit drawn directly from the chip itself.

[janneman]

Hmm. looks OK. Not having a cap in the feedback loop explains the offset, but it's not too bad. Are you sure the supply voltages at the chip are OK?

jan Didden

[hughmon]

One possibility that comes to mind is an amplitude dependent oscillation. Basically as the output signal goes up, at some amplitude it reaches a sufficient magnitude to feedback to the input and osc. occurs. This tends to be a layout kind of thing, but can be bandaided with a capacitor to ground at the non-inverting input (if interested, try a 470pF cap to start with). Sometimes a 100 ohm resistor is needed between the actual input pin of the chip and the node of the cap and input resistor. Don't know if this will help or not, but it's what I'd try at this point.

[Adam M.]

Jan, I have tested the voltages on the power supply with reference to 0V at the middle of the transformer secondaries, and I get +-37.2V within a small margin on each rail, I checked this with the chip off before I hooked it up to make sure it was ok, and again with the chip in place. I have not checked the current it delivers.

hughmon, I'll give the cap from non-inverting input to ground a try and see if that helps.

Thank you for your time. If after some more troubleshooting I can't get it to work, I'll grab a digital camera and post a picture of the layout.